Unit 3 Lecture 2: Determinants of Glomerular Filtration

Question 1

What dictates whether glomerular filtration occurs?

Answer 1

1. Pressure of glomerular capillaries (biggest pressure) (55mmHg)
2. Bowman space pressure (small due to confined space) (15mmHg)
3. Plasma protein osmotic pressure (most similar to the capillary vasculature) (30mmHG)

Question 2

How is capillary pressure (vasculature system) different from glomerular capillary pressure?

Answer 2

• Capillary pressure in the vasculature system depends on tube length and it is involved in both filtration and reabsoption
• Glomerular capillary pressure is higher and only involved in filtration

Question 3

How is bowman’s space pressure different from interstitial space fluid pressure?

Answer 3

• Interstitial fluid space pressure = 1mmHG (bigger space)
• Bowman’s space pressure = 15mmHg (smaller space)

Question 4

Why is there no osmotic pressure dictated by the Bowman’s space?

Answer 4

It is non-existent because there should be no plasma proteins in the bowman’s space but there should be in the glomerulus (osmotic pressure dictated by Glomerular capillaries)

Question 5

Net glomerular filtration pressure formula

Answer 5

PGC - PBS- πGC = 10mmHg

Question 6

What is making glomerular filtration rate (GFR) so high? Determinants of net GFR

Answer 6

1. Net filtration pressure (largely controlled by Pgc)
2. Kf (Filtration coefficient) - SA of capillaries and glomerular permeability (pore sizes)

Filtration can be up to 125mL/min (remember most of it is reabsorbed)

Question 7

What is the filtration coefficient (Kf)?

Answer 7

Depends on the surface area of the glomerular capillary, which is a filter & it depends on the size of the pores which indicate the permeability (how much goes through the glomerulus)

Note: Kf dictates the rate of glomerular filtration (small SA and not much pores = less filtration)

Question 8

Why is the filtration coefficient a coefficient?

Answer 8

The surface area and pore size are constant (numbers never change from what we see)

Question 9

If arterial blood pressure in the afferent arteriole increased what would that do to the glomerular capillary blood pressure (Pgc) & glomerular filtration rate (GFR)

Answer 9

↑BP ⇒ ↑PGC ⇒ ↑GFR
* Arteriole dictates the blood flow to the glomerulus by smooth muscle contraction

Direct effect

Question 10

What happens if we vasoconstrict the afferent arteriole without changing the MAP?

Answer 10

• Restricts the amount of blood flowing & therefore how much the glomerulus filters decreases

Note: Changing local control of blood flow can dictate GFR

Question 11

What happens if we vasodilate the afferent arteriole without changing the MAP?

Answer 11

If we stretch the afferent arteriole, this increases blood flow which increases glomerular capillary blood pressure & therefore glomerular filtration rate

Note: Changing local control of blood flow can dictate GFR

Question 12

What is the impact on Glomerular capillary pressure when we vasoconstrict the efferent arteriole? What about the effect on GFR?

Answer 12

• This will increase glomerular capillary pressure
• High Gcp leads to high Glomerular filtration rate (↑GFR)

Question 13

What is the impact on Glomerular capillary pressure when we vasodilate the efferent arteriole? What about the effect on GFR?

Answer 13

• More expanded glomerulus means less pressure on it
• Less pressure on Gcp means less GFR (↓GFR)

Question 14

When we have a change in MAP, how does it affect GFR? How do we maintain a steady GFR?

Answer 14

An increase in MAP, increases glomerular capillary pressure, which increases GFR
* Excess amounts of urine and fluids will be constantly excreted

Maintaining a steady GFR requires afferent/efferent arteriole vasoconstriction/vasodilation

Question 15

What local regulatory mechanisms maintain GFR when MAP changes

Answer 15

1. Flow auto-regulation
2. Juxtaglomerular feedback

Question 16

Explain how flow auto-regulation maintains GFR when MAP changes

Answer 16

• Smooth muscle in afferent arteriole stretches in response to increased MAP
• Upon stretching Pgc and GFR increase
• When the stretch is detected, the myogenic mechanism induces vasoconstriction on the afferent arteriole
• Pgc and GFR return to normal

Question 17

What is Juxtaglomerular feedback dependent on? Where does it occur?

Answer 17

It is sensitive to NaCl in the tubule
* Occurs at juxtaglomerulus apparatus

Question 18

What is the juxtaglomerular apparatus?

Answer 18

It is the space where the distal tubule meets with the connection between the afferent and efferent arteriole

Question 19

What makes up the juxtaglomerular apparatus?

Answer 19

1. Macula densa - Specialized epithelium cells around the distal tubule; sense [NaCl] in the distal tubule
2. Granular cells - Situated on the wall of the afferent arteriole adjacent to the macula densa; specialized endothelium cells

Question 20

How do the macula densa and granular cells function?

Answer 20

• Macula densa secretes factors that affect the afferent arteriole diameter
• Granular cells produce factors in response to the Macula densa and what it has secreted & sensitive to SNS

REMEMBER: Macula densa only activates when it senses NaCl (sensitive to this)

Question 21

What dictates Juxtaglomerular feedback? Macula densa or granular cells?

Answer 21

Macula Densa

Question 22

Explain the feedback loop of the juxtaglomerular apparatus when there is increased [Nacl] in the distal tubule

Answer 22

• Increased distal tubule [NaCl] causes Macula densa to secrete its factors
• Adenosine is specifically secreted to allow for vasoconstriction of the afferent arteriole
• This will cause blood flow to the glomerular capillary to decrease which means Gcp decreases and GFR too

Note: Purpose of this is because of too much sodium so we want to decrease absorption of NaCl into the distal tubule

Question 23

Explain the feedback loop of the juxtaglomerular apparatus when there is decreased [Nacl] in the distal tubule

Answer 23

• Decreased distal tubule [NaCl] which make Macula Densa secrete factors
• Prostaglandin is a factor which will increase the diameter of the afferent arteriole (vasodilation)
• This increases glomerular capillary pressure and glomerular capillary blood flow which also increases GFR